Image scanners are commonly used in offices or homes for scanning for example documents, photographs or film. For example, the scanned images are stored into a computer. Alternatively, the scanned images can be directly printed from or stored into multifunction peripherals.
When an image scanner is operated, an image sensing module such as a charge couple device (CCD) or contact image sensor (CIS), is employed to produce electronic signals of images. Nowadays, since the contact image sensor has less volume than the charge couple device, many manufactures make efforts in developing the technologies of the contact image sensor applied to the scanner.
CIS modules are generally classified as two major types as shown in FIGS. 1 and 2.
Referring to FIG. 1, a schematic view of the first type CIS module is shown. In FIG. 1, the first type CIS module 100 comprises a cold cathode fluorescent lamp 101 and three rows of image sensors. The red image sensors comprise a plurality of sensors R1–RN. The green image sensors comprise a plurality of sensors G1–GN. The blue image sensors comprise a plurality of sensors B1–BN. The cold cathode fluorescent lamp 101 is used as a light source to project light onto the object to be scanned. The light reflected from the scanned object is received by these three rows of image sensors so as to generate image signals of the scanned object. As known, the red image sensors R1–RN, the green image sensors G1–GN and the blue image sensors B1–BN are capable of generating red, green and blue image signals, respectively.
Referring to FIG. 2, a schematic view of the second type CIS module is shown. In FIG. 2, the second type CIS module 200 comprises one row of image sensors S1–SN and three light emitting diodes (LEDs) for emitting three different colors. For these light emitting diodes, the terms “LED R”, “LED G” and “LED B” indicate red LED, green LED and blue LED, respectively. During scanning operation, only one of these LEDs is turned on at the same time and thus red, green and blue image signals of the scanned object are successively generated. Generally, the red LED is turned on to use as a light source to project red light onto the object to be scanned and thus the image sensors Si–SN generate red image signals of the scanned object. Successively, the green LED is turned on to generate green image signals of the scanned object. Afterward, the blue LED is turned on to generate blue image signals of the scanned object.
In views of a comparison between these two types of conventional CIS modules, the second type CIS module shown in FIG. 2 is more cost-effective than the first type CIS module in FIG. 1 because the second type CIS module needs only one row of image sensors. In contrast, for the first type CIS module in FIG. 1, three rows of image sensors are required.
Although the second type CIS module shown in FIG. 2 is advantageous due to its cost effectiveness, such a CIS module still has some drawbacks in use. For example, the second type CIS module shown in FIG. 2 utilizes three LEDs as light sources to emit light with three different colors. Since each scan line only receives the light emitted from a single LED at the same time, the brightness of the light source is often insufficient for the image sensors. This insufficient brightness leads to an unsatisfactory signal-to-noise ratio of the image signal. In order to overcome this problem, conventionally, high-brightness LEDs are used to emit intense light. This solution, however, results in an increased cost of the CIS module. Another solution is to extend the exposure time of the image sensors. In other words, for each scan line, the time period for the LED to project light thereon is increased such that the light amount received by the image sensors is increased. In such way, however, the scanning operation is time-consuming and the scanning speed is reduced.